from: http://www.helmets.org/replace.htm

Most manufacturers now recommend that helmets be replaced after five years, but some of that may be just marketing. (Bell now recommends every three years, which seems to us too short. They base it partially on updating your helmet technology, but they have not been improving their helmets that much over three year periods, and we consider some of their more recent helmets to be a step backwards, so we would take that with a grain of salt.) Deterioration depends on usage, care, and abuse....
Somebody is spreading rumors that sweat and Ultraviolet exposure will cause your helmet to degrade. Sweat will not do that. The standards do not permit you to make a helmet that degrades from sweat, and the EPS, EPP or EPU foam is remarkably unaffected by salt water. Your helmet will get a terminal case of grunge before it dies of sweat. UV can affect the strength of the shell material, though. Manufacturers put UV inhibitors in the plastic for their shells that control UV degradation. If your helmet is fading, maybe the UV inhibitors are failing, so you might consider replacing it. Chances are it has seen an awful lot of sun to have that happen....
In sum, we don't find the case for replacing a helmet that meets the ASTM or Snell standards that compelling if the helmet is still in good shape and fits you well.